Re: A thread on grub-bug could need attention

[Thomas Schmitt]

Hi,

Michel Bouissou wrote:
> I'm happy to report that the USB stick I made today with
> grub-mkrescue-sed.sh does boot alright on tested machines.

So GRUB itself is ok with the machine. GPT and MBR partitions alike.


> The HP tested machine has, AFAIK, no CSM "legacy" boot mode at all and I
> believe it doesn't boot anything but UEFI.

It booted the tails ISO which has no EFI boot equipment but only
an ISOLINUX El Torito boot image for BIOS and an ISOLINUX isohybrid MBR
with machine code which BIOS will execute when presented on USB stick.

The machine's firmware probably falls back to legacy mode when detecting
BIOS stuff but no EFI partition.

-------------------------------------------------------------------------

What only do our users at the distro ISO projects miss which grub-mkrescue
does right ?

Is it simply the invalid GPT which Matthew J. Garrett introduced
to get some of his test machines booting ? (Or did he introduce the MBR
partition without noticing that this invalidates GPT ?)
See
  "Anatomy of a Fedora 17 ISO image"
  https://mjg59.dreamwidth.org/11285.html
preceeded by
  "Further adventures in EFI booting"
  https://mjg59.dreamwidth.org/4957.html
The EFI images, which he and nearly all others use, are made with GRUB2.

grub-mkrescue does a similar stunt about x86 machine code without
side effects and the Block0 of an Apple Partition Map. But it does not
combine a non-protective MBR with GPT.

Michel - on his way to India - could zeroize 512-byte block 1 of the
Debian Live ISO and then try to boot it from USB stick

  dd if=/dev/zero conv=notrunc bs=512 seek=1 count=1 of=...image.or.stick...

where "...image.or.stick..." mabe either something like debian-live-*.iso
or /dev/sdc.
This would further deface the GPT and possibly de-confuse the firmware.

Michel, i will try to remember to remind you, when you are back. :))

-------------------------------------------------------------------------
The partition tables of modified and vanilla grub-mkrescue ISOs:

This is the modified one from grub-mkrescue-sed.sh :

> address@hidden:/# xorriso -indev stdio:/dev/sdb -report_el_torito plain 
> -report_system_area plain
> ...
> El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
> El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4        1487
> El Torito boot img :   2  UEFI  y   none  0x0000  0x00   5760        7154
> El Torito img path :   1  /boot/grub/i386-pc/eltorito.img
> El Torito img opts :   1  boot-info-table grub2-boot-info

We see that it's with EFI equipment.
(LBA counts blocks of 2048 bytes.  Ldsiz counts blocks of 512 bytes.
 Blame IBM and Phoenix, not me.)


> MBR partition table:   N Status  Type        Start       Blocks
> MBR partition      :   1   0x80  0x00           64        28552
> MBR partition      :   2   0x00  0xef        28616         5760

(Here all block numbers are counted with 512 bytes per block.)

By the options manipulation of grub-mkrescue-sed.sh the EFI partition
image was appended after the mountable ISO 9660 partition. Other than
with vanilla grub-mkrescue both partitions are marked in the MBR partition
table.
Partition 1 has the Boot/Active flag set, because some firmwares ignore
media which do not have it on any partition.
Partition 2 is the EFI partition. The same bytes as the El Torito UEFI
boot image. (7154 * 2048 / 512 = 28616)

Vanilla grub-mkrescue would rather look like:

  El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
  El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4        4047
  El Torito boot img :   2  UEFI  y   none  0x0000  0x00   5760          85
  El Torito img path :   1  /boot/grub/i386-pc/eltorito.img
  El Torito img opts :   1  boot-info-table grub2-boot-info
  El Torito img path :   2  /efi.img
  ...
  MBR partition table:   N Status  Type        Start       Blocks
  MBR partition      :   1   0x00  0xee            1        34779
  GPT                :   N  Info
  GPT disk GUID      :      04a5adf35d1adb4382bf8300bebe08a1
  GPT entry array    :      20  176  separated
  GPT lba range      :      64  34734  34779
  GPT partition name :   1  4700610070003000
  GPT partname local :   1  Gap0
  ...
  GPT partition name :   2  
450046004900200062006f006f007400200070006100720074006900740069006f006e00
  GPT partname local :   2  EFI boot partition
  GPT partition GUID :   2  04a5adf35d1adb4382bd8300bebe08a1
  GPT type GUID      :   2  28732ac11ff8d211ba4b00a0c93ec93b
  GPT partition flags:   2  0x1000000000000001
  GPT start and size :   2  340  5760
  GPT partition path :   2  /efi.img
  ...
  GPT start and size :   4  34132  600

plus maybe

  APM                :   N  Info
  APM block size     :      2048
  ...

Other than with the mbr_only version, the EFI partition is a data file
inside the ISO 9660 filesystem: /efi.img

247 lines of explanation of the report format are available with
  xorriso -report_system_area help -report_el_torito help | less


Have a nice day :)

Thomas